FIG. 29 is a block diagram showing an example of configurations of conventional image display devices. The operation of a conventional image display device will be described below with reference to FIG. 29. This image display device is configured with an input image processing means 1 and an image display means 3, as shown in FIG. 29.
Referring to FIG. 29, image data Ri, Gi, and Bi that are composed of three color (RGB) data to be inputted to the image display device are inputted to the input image processing means 1. The inputted image data Ri, Gi, and Bi are subjected to input image processing, which will be described hereinafter in connection with the input image processing means 1, and then outputted as image data R1, G1, and B1 composed of three color data. The image data R1, G1, and B1 outputted from the input image processing means 1 are sent to the image display means 3. In the image display means 3, in response to the corresponding image data value, each pixel emits a light for image display. As an example of the image display means, there is a liquid crystal panel or CRT.
FIG. 30 is a block diagram showing an example of the configuration of the input image processing means 1 in FIG. 29. Referring to FIG. 30, the input image processing means 1 is configured with a pixel number transforming means 101, color transforming means 102, and gradation transforming means 103.
The operation of the input image processing means 1 will be described hereinafter. Image data Ri, Gi, and Bi inputted to the input image processing means 1 are inputted to the pixel number transforming means 101 and subjected to pixel number transformation so as to match the display pixel number in the image display means 3, and then outputted.
The output from the pixel number transforming means 101 is inputted to the color transforming means 102 and subjected to color transformation processing in consideration of the color reproduction characteristics of the image display means 3. Performing this color transformation processing realizes display of a desirable color reproduction in the image display means 3.
The output from the color transforming means 102 is inputted to the gradation transforming means 103 and subjected to gradation correction processing in response to the characteristics of the image display means 3, and then outputted as image data R1, G1, and B1. The pixel number transforming means 101, color transforming means 102, and gradation transforming means 103 may be configured with hardware or software.
Description will now be given of the relationship between the size of the image data R1, G1, and B1 inputted to the image display means 3, and the color (light) displayed on the image display means 3. Let X1, Y1, and Z1 denote tristimulus values based on the CIE XYZ calorimetric system of color (light) displayed on the image display means 3, when image data R1, G1, and B1 are inputted to the image display means 3 in a situation where there is no influence of external light (hereinafter referred to simply as “tristimulus values”). Assume that the image display means 3 is an image display means in which the relationship between the size of image data R1, G1, and B1 to be inputted and the tristimulus values X1, Y1, and Z1 of color (light) to be displayed can be expressed in the following equation (1):                               [                                                    X1                                                                    Y1                                                                    Z1                                              ]                =                                            [                                                                    axr                                                        axg                                                        axb                                                                                        ayr                                                        ayg                                                        ayb                                                                                        azr                                                        azg                                                        azb                                                              ]                        ⁢                                                   [                                                            R1                                                                              G1                                                                              B1                                                      ]                    +                      [                                                            Xbk1                                                                              Ybk1                                                                              Zbk1                                                      ]                                              (        1        )            where Y1 is a value corresponding to a luminance.
In equation (1), axr, ayr, azr, axg, ayg, azg, axb, ayb, azb; and Xbk1, Ybk1, Zbk1, are values that depend on the characteristics of the image display means 3. In particular, Xbk1, Ybk1, and Zbk1 are tristimulus values of color (light) displayed on the image display means 3 when the image display means 3 displays black in a situation where there is no influence of external light, that is, when R1=G1=B1=0. Here, axr, ayr, azr, axg, ayg, azg, axb, ayb, and azb can be expressed in the following equation (2):axr=0.4124, axg=0.3576, axb=0.1805, ayr=0.2126, ayg=0.7152, ayb=0.0722, azr=0.0193, azg=0.1192, azb=0.9505  (2)
Image data R1, G1, and B1 to be inputted to the image display means 3 are integers and have values in the range expressed in the following equation (3):0≦R1≦100 0≦G1≦100 0≦B1≦100  (3)
Theoretically, all tristimulus values in displaying black, Xbk1, Ybk1, and Zbk1, should be “0”, however, they have in fact values larger than “0”. Further, let X2, Y2, and Z2 denote tristimulus values of a reflected light caused by that external light irradiates the surface of the image display means 3 and the external light is reflected from the surface of the image display means 3. In this case, tristimulus values X3, Y3, and Z3 of light received by the eyes of a viewer who views the image display means 3 can be expressed by the sum of the tristimulus values X1, Y1, and Z1 of color to be displayed on the image display means 3 by input signals R1, G1, and B1, and the tristimulus values X2, Y2, and Z2 of the reflected light. That is, X3, Y3, and Z3 can be expressed in the following equation (4). The viewer seems as if the color expressed by X3, Y3, and Z3 were displayed on the image display means 3.                               [                                                    X3                                                                    Y3                                                                    Z3                                              ]                =                                            [                                                                    X1                                                                                        Y1                                                                                        Z1                                                              ]                        +                          [                                                                    X2                                                                                        Y2                                                                                        Z2                                                              ]                                =                                                    [                                                                            axr                                                              axg                                                              axb                                                                                                  ayr                                                              ayg                                                              ayb                                                                                                  azr                                                              azg                                                              azb                                                                      ]                            ⁢                                                           [                                                                    R1                                                                                        G1                                                                                        B1                                                              ]                        +                          [                                                                                          Xbk1                      +                      X2                                                                                                                                  Ybk1                      +                      Y2                                                                                                                                  Zbk1                      +                      Z2                                                                                  ]                                                          (        4        )            
From equation (4), Xbk1+X2, Ybk1+Y2, and Zbk1+Z2 are tristimulus values when displaying black on the image display means 3, taking the influence of external light into consideration. From equation (4), the variations in value of the tristimulus values Xbk1, Ybk1, and Zbk1 when displaying black in a situation where there is no influence of external light, and the variations in value of the tristimulus values X2, Y2, and Z2 of the reflected light of external light, exert the same influence on the tristimulus values X3, Y3, and Z3 of light received by the viewer's eyes. Therefore, the following is the instance that the values of Xbk1, Ybk1, and Zbk1 are fixed and the values of X2, Y2, and Z2 vary due to the influence of external light. The same concept is applicable to the instance that the values of Xbk1, Ybk1, and Zbk1 vary. Here, let Xbk1, Ybk1, and Zbk1 be values expressed in the following equation (5):Xbk1=1Ybk1=1Zbk1=1  (5)
FIG. 31 is an explanatory diagram showing in table the relationship between R1, G1, and B1 inputted to the image display means 3 and tristimulus values X3, Y3, and Z3 of color (light) received by the viewer's eyes in a situation where there is no influence of external light, i.e., when X2=Y2=Z2=0. Specifically, FIG. 31 shows the instance that the relationship of R1=G1=B1 holds, i.e., an achromatic data is inputted to the image display means 3.
Consider now the instance that there is no influence of external light, by referring to FIG. 31. If there is no influence of external light, X2=Y2=Z2=0. When the maximum values of image data R1, G1, and B1, i.e., 100, 100, and 100, are inputted to the image display means 3, the tristimulus values of color (light) received by the viewer's eyes are X1=96.05, Y1=101, and Z1=109.9, in a situation where there is no influence of external light. On the other hand, when the minimum values of image data R1, G1, and B1, i.e., 0, 0, and 0, are inputted to the image display means 3, the tristimulus values of color (light) received by the viewer's eyes are X1=1, Y1=1, and Z1=1, in a situation where there is no influence of external light.
In FIG. 31, the ratio of Y3 that corresponds to luminance in the tristimulus values of color (light) received by the viewer's eyes when R1, G1, and B1 are inputted to the image display means 3, to Y3 when R1=100, G1=100, and B1=100 (when displaying white), is indicated as a ratio to white (Y/Ymax). The viewer seems that the image displayed on the image display means 3 has a larger contrast and more excellent visibility as the value of ratio to white is smaller to each image data.
FIG. 32 is a graph showing the relationship between image data R1, G1, and B1 inputted to the image display means 3, and a luminance stimulus value Y3.
Description will next be given of image display in an image display means 3 of a conventional image display device when the device is used under environment where there is the influence of external light.
FIG. 33 is an explanatory diagram showing in table the relationship between R1, G1, B1, and the tristimulus values X3, Y3, Z3 of color (light) received by the viewer's eyes in a situation where there is the influence of external light. Specifically, FIG. 33 shows the instance that the relationship of R1=G1=B1 holds, i.e., an achromatic data is inputted to the image display means 3.
Here, suppose that the tristimulus values of a reflected light of external light on the surface of the image display means 3 are X2=9.505, Y2=10, and Z2=10.89. When the maximum values of R1, G1, and B1, namely, 100, 100, and 100, are inputted to the image display means 3, the tristimulus values of color (light) received by the viewer's eyes are X3=105.555, Y3=111.000, and Z3=120.790. On the other hand, when the minimum values of R1, G1, and B1, i.e., 0, 0, and 0, are inputted to the image display means 3, the tristimulus values of color (light) received by the viewer's eyes are X3=10.505, Y3=11.000, and Z3=11.890.
Also in FIG. 33, the ratio of Y3 that corresponds to luminance in the tristimulus values of color (light) received by the viewer's eyes when R1, G1, and B1 are inputted to the image display means 3, to Y3 (Ymax) when R1=100, G1=100, and B1=100 (when displaying white), is indicated as a ratio to white (Y/Ymax). The values obtained when there is the influence of external light are large as a whole than when there is no influence of external light, as shown in FIG. 31. That is, when there is the influence of external light, the viewer seems that the image has a small contrast and poor visibility.
FIG. 34 is a graph showing the relationship between image data R1, G1, and B1 inputted to the image display means 3, and a luminance stimulus value Y3. In FIG. 34, a continuous line represents the instance that there is the influence of external light, and a dotted line represents the instance that there is no influence of external light.
In order to suppress a drop in contrast due to the influence of external light, it can be considered to increase the brightness of display on the image display means 3 in a situation where there is the influence of external light. For instance, doubling the brightness of display on the image display means 3 doubles tristimulus values X1, Y1, and Z1 of color (light) displayed on the image display means 3.
FIG. 35 is an explanatory diagram showing in table the relationship between R1, G1, B1, and the tristimulus values X3, Y3 and Z3 of color (light) received by the viewer's eyes in a situation where the brightness of display on the image display means 3 is double that of the above instance, and there is the influence of external light. Specifically, FIG. 35 shows the instance that the relationship of R1=G1=B1 holds, i.e., an achromatic data is inputted to the image display means 3. Again, suppose that the tristimulus values of a reflected light of external light on the surface of the image display means 3 are X2=9.505, Y2=10, and Z2=10.89, as in the instance of FIG. 33.
Also in FIG. 35, the ratio of Y3 that corresponds to luminance in the tristimulus values of color (light) received by the viewer's eyes when R1, G1, and B1 are inputted to the image display means 3, to Y3 (Ymax) when R1=100, G1=100, and B1=100 (when displaying white), is indicated as a ratio to white (Y/Ymax). As compared to the instance in FIG. 33, doubling the brightness of display on the image display means 3 makes the values of ratio to white approach the ratios to white in FIG. 31 showing the instance that there is no influence of external light. However, the values are still large as compared to FIG. 31. There is also such technical background that it is very difficult to double the brightness of display on the image display means 3, due to problems of cost, problems of power consumption, and problems of useful life.
Thus, the conventional image display device suffers from the problem that when there is the influence of external light or when the luminance in displaying black has a large value due to the characteristics of the image display means, a ratio to white (Y/Ymax), which is a ratio of a luminance displayed for each image data to a luminance in displaying white, is considerably large and the viewer seems that the image has a small contrast and poor visibility.
There is also the problem that a mitigation of the increased ratio to luminance in displaying white by increasing the brightness of display on the image display device results in poor improvement effect, though this is very difficult due to problems of cost, problems of power consumption, and problems of useful life.